In recent years, broadband communication lines such as ADSL and the like have prevailed. An IP telephone service that packetizes and transmits audio data by utilizing a high-speed transmission feature of such broadband line has become widespread rapidly.
Against the background of such situation, a method of making high-speed image communications in real time by the same operation as a conventional facsimile system using the IP telephone network has been proposed.
In the IP telephone network, when a telephone number is input at a connected terminal, a session request message containing a destination telephone number is transmitted to an SIP (Session Initiation Protocol) server. Upon reception of the session request message, the SIP server sends an inquiry to an ENUM server to acquire an URI (Universal Resource Identifiers) as an identifier corresponding to the telephone number. Next, the SIP server acquires an IP address corresponding to the URI from a DNS (Domain Name System) server. After that, packets transmitted from the terminal are sent to a destination terminal having that IP address.
When a terminal recognizes the IP address of the partner terminal based on the IP address in a packet transmitted from the SIP server, the source terminal can transmit IP packets peer to peer to the destination terminal.
Hence, an image communication terminal requests a server to send IP address information corresponding to a destination telephone number prior to transmission of an image frame, appends the destination IP address received from the server to the image frame, segments a loaded image into a plurality of frames, appends the destination IP address to each segmented frame, and transmits the frames, thus implementing real-time high-speed image transmission.
However, in the above system, the terminal must transmit a session request message to the SIP server upon transmitting an image. For this reason, a heavy load is imposed on the SIP server.
By contrast, in one implementation method of an Internet telephone using ISDN, information indicating whether or not Internet telephone communication is allowed is stored in a memory in correspondence with telephone numbers. However, a destination IP address is acquired from an access point of the destination, and IP packets cannot be directly transmitted to the destination.
In another method, when a gateway stores IP addresses and telephone numbers in a memory in association with each other, a plurality of terminals connected to the gateway can directly exchange IP packets. However, in this case, when a terminal is connected to the gateway, its IP address and telephone number must be registered in association with each other.
In this manner, peer-to-peer communications cannot be made by designating the IP address of a partner terminal by only inputting a telephone number without any special operation, connection to the server, or the intervention of the gateway.